Goodness me how complicated can the points system get? Elena your explanation sounds great but I don't fully understand it but I've copied it and shall await events and hope it becomes clearer eventually
I don't fully understand it either! I was just querying whether the DC WG play off counts as a 500 tournament or not. Looking at what's happened with other players I think probably not, so agree with Dex - the 0 point penalty is because Andy will be at least one 500 short and will stay on for a year. Sorry if I've confused people!
Ruthie - top 30 players have to play in 4 x 500 tournaments (MC can be included for some reason, and one has to be after US Open) in order to count the 6 best results from any 500 and 250s. I think it used to be rigidly that 4 x 500 and 2 x 250 were counted, but I'm not sure. If you look at the rankings breakdown of lots of players below the top few, they have played enough 500s, but gained more points from 250s. Gasquet is a good example http://www.atpworldtour.com/Tennis/Players/Top-Players/Richard-Gasquet.aspx?t=rb
(Edit to include correct link!)
Any extra points beyond the top 6 go into "non-countable" but these can be moved around at any time to replace others that are lower or drop off etc. Ferrer had a couple of 250 wins in his non-countables earlier this year, which came in handy when he didn't defend his points at Barcelona and elsewhere.
The 0 point penalty is really only relevant if a player has extra points from other tournaments, as the penalty stays in force for a year and can't be replaced by a better result elsewhere. Djoko and Andy don't have any extra, so it doesn't really make any difference at the moment. Nadal and Fed both had point penalties at the end of last year and have just a few points they can't count at the moment - not enough to make any difference to anything though.
If Andy plays more 500s next year he may not be able to count all the points from them immediately - I think he can replace his 90 points from MC if he scores more elsewhere (including Bangkok). If he doesn't play more tournaments the penalty won't make much difference!
Hope that makes some sense!
The rules are all here, and explain when the 0 point penalty is given.http://www.atpworldtour.com/Rankings/Rankings-FAQ.aspx
Re the O2 qualification cut-off - they must work out how the remaining points available can be divided as the tournaments go by, and at which point a player can't drop below 8 before the WTF. The more points Nadal wins, the fewer there are for anyone else!